A substantial portion of modern-day technology depends for its operating power on commercially supplied AC power sources, and a significant number of electronic systems require extremely reliable sources of power. Examples of electronic systems needing reliable power include computers, data processors, process controllers, and communications and laboratory equipment.
In general, the available commercial power supplied by utilities arriving at the user's location (i.e., wall plug) is not sufficiently reliable to meet the power needs of such equipment. Commerical power is sometimes subject to complete outages, or in other words, complete failure at the power source, these conditions being known as blackouts. More often, due to inadequate capacity and increasing load demands, commercial power is subject to a condition known as "brownout", sags which normally occur during peak demand periods and usually are typically represented by a 3% to 8% drop in magnitude of the available voltage. Commerical power is very frequently subject to magnitude and reactive instabilities causing irregular voltage waveforms due to transients induced by the action of various customers who subject the system to sudden electrical loads, power line switching equipment, or nearby high frequency "noise" generating equipment such as the motors of small appliances or hand tools, arc producing equipment such as fluorescent lights or switching-type DC power supplies. Power may be lost or flicker due to storms, accidents, or utility switching.
Power level variations such as described above can significantly affect equipment, and may, in some instances, damage that equipment. For example, in communication circuitry, even a transient interruption or surge may cause undetected errors in data or control signals or cause damage that is not readily detectable or obvious. Specifically, to operate properly, a computer requires a precisely regulated, continuous power signal. The fluctuations it can tolerate from a power source are extremely limited. The computer can probably tolerate momentary spikes and dips in the voltage if the duration is only a few milliseconds. It can probably also tolerate a slight brownout for a short period of time such as 3-100 milliseconds. Should a voltage drop last beyond a certain period, it is possible that the computer could malfunction or shut down. In certain instances, there is a very definite possibility that processing errors may occur requiring partial program reruns. In instances of severe brownout or drop out, the computer may go into total shutdown and may terminate operations with a possibility of component damage and/or adverse effects on the integrity of the stored data and program. There are also a large number of separate occurrences where very large voltage transients may appear on the line, on any combinations of phase conductors, neutral, and/or ground.
It is, therefore, apparent that for safe, reliable operation a computer needs a source of continuous, regulated power having very stable characteristics. Since the power normally available at the user's location does not possess the necessary stability and lack of transients for safe operation of computers, it is customary to supply the power to computers with uninterruptible power supplies which are essentially external to the computer. Uninterruptible power supplies guarantee the continuity of power regardless of the performance of the primary commercial AC power source upon which a customer relies. Generally, uninterruptible power supplies include plural sources of power which usually operate in conjunction with each other to provide a continuous power output to some load to be energized. The plural sources generally include a commercial AC power source and an auxiliary independent source of power to supplement or substitute for the commercial AC power as required in order to supply the necessary continuous and stable power input to the load to be energized.
There are several methods of providing an uninterruptible power supply. One simple method is to connect a charger and rectifier to the commercial AC power source. The rectified output is connected in parallel with a reserve battery-type power source and both sources are used to drive an inverter circuit from which the power signal to energize the computer is derived. This power supply arrangement is complex and inefficient due to the need to output an alternating current. It is also inefficient and cumbersome to regenerate an alternating current (usually 60 cps) which is subsequently again converted to direct current. Furthermore, there is usually no redundancy to provide power to the load should the inverter fail. It is generally recognized that inverters are fairly reliable, but they are generally incapable of handling rapid load change demands which generally result in overcurrent, short circuits, or in-rush current conditions. A rapid load transfer can create signal disturbances which will destroy the inverter or interfere with operation of the computer components.
To avoid these problems, uninterruptible power supplies have been designed where the primary commercial power source and the reserve power source are connected in parallel. Both the primary power source and the reserve power source are continuously operated and both sources contribute to the energization of the load. This is a completely redundant system and should either power source fail, the results are not apparent to the load which is continuously energized. Such an uninterruptible power supply system may use a ferroresonant transformer with two input primaries which are coupled to energize a single secondary. Through the use of properly designed high reluctance shunts, the two power sources do not transmit power to each other. Both power sources cooperate to share the load's power needs. The disadvantage of this particular arrangement is the expensive transformer design of a ferroresonant transformer having carefully designed high reluctance shunts and symmetrical construction to permit the two power sources to share the load. Both of the above-described systems are also rendered somewhat inefficient and unduly complex on an overall system basis because of the regeneration of an (60 cps) alternating current.
Another form of the just-discussed parallel power source design includes batteries connected in parallel using a diode matching system. In such a system diodes are used to prevent the output current of one of the power sources from flowing into the other of the power sources when the output voltage of one of the power sources is greater than that of the other.
In the case where two power sources are operated in parallel as described above, the power source which has a greater output voltage supplies substantially 100% of the load current. Under this condition, should the other power source be deactivated, the load is not affected because the first-mentioned power source still supplies output current to the load. On the other hand, should the first-mentioned power source be deactivated, the other power source starts supply current to the load. Thus, in both cases, the load is constantaly supplied with load current.
As is clear from the above description, the parallel power source diode matching system is advantageous in that the number of components required is relatively small and accordingly the arrangement is simple. However, the operation of the parallel power source diode matching system has been disadvantageous for the following reasons:
(1) In practice, the difference between the output voltages of the parallel power sources will never become zero. Therefore, it is difficult to maintain load balance between the power sources; that is, the load current is always supplied by only one of the power sources. Accordingly, the temperature of the power source supplying the load current increases, such that the power source itself (and accordingly the power source system) is degraded in reliability. Since the reliability of the power source to be parallel-operated is increased, the reliability of the system is not improved.
(2) In the case where the parallel operation is carried out in order to increase the output capacity, the load balance is not sufficient to maintain both power sources in their conductive states. As a result, the load current is supplied by only one of the power sources, and it becomes necessary to increase the capacity of the transistor which forms the power source. Thus, it is impossible to decrease the capacity of the transistor by employing an overcurrent protection system which provides a particularly beneficial output voltage vs. load current characteristic for the power source.
(3) The load balance is insufficient (as described herein). Therefore, when the power sources switch such that a source which was previously non-conductive is rendered conductive, the output voltage drops significantly during the switch.
(4) Because of the characteristics of the matching diodes, the output voltage depends upon either the load current or the ambient temperature; that is, it is difficult to maintain the output voltage at a constant level with a high degree of accuracy.
An uninterruptible power supply arrangement which permits use of a less expensive power coupling arrangement is commonly known as the "transfer type" of uninterruptible power supply. Generally, a commercial AC line power source and a DC voltage energized inverter power source are connected in parallel to a switching mechanism which alternately couples one or the other of the two power supplies to a load to be energized. This power supply design advantageously eliminates the need for an expensive ferroresonant transformer and provides redundancy to provide a substantially uninterruptible power to the load. In addition, the auxiliary power source comprising the inverter should be synchronized in frequency with the AC power line signal which requires complicated synchronizing circuitry or else high frequency transients may result. The switching action must be sufficiently fast to handle the transition of a load from a failed AC power line to the reserve power source or inverter circuit without inducing damaging transient signals into the circuit. Because the power switch must commutate a large amount of current, its switching time is relatively slow in comparison to the electronic circuitry to which it is supplying power. In addition, the complexity of such a power switch also makes it fairly expensive. This system suffers from still another problem in that the reliability of the entire system is based in part on a single power switch. In fact, the reliability of a single power supply is traded for the reliability of a single power switch. The switching must disconnect the failed power source so it does not become a load for the active power source. Additionally, should the inverter fail, it would only be discovered at the time it is needed most, i.e., at the moment of a power transfer.
In yet another uninterruptible energy system, an energy store is provided between the load and the normal energy supply system. This store receives energy from the main supply system and gives it up to the loads during normal opeation, i.e., so that in effect, the store can be regarded as the energy supply for the load at all times, bur normally the store itself is being replenished by the main supply. If the main supply fails, the store can nevertheless maintain the supply of energy to the load (until, of course, the main supply has been out of action for so long at any one time that the store becomes empty). Due to this "interposition" of energy sources, the loads are sometimes considered completely independent of any disturbances, e.g., voltage reductions or voltage rises, and distortion, emanating from the main energy supply system. However, normal mode (transformer) coupling and triggered static bypass switches may still pass power line disturbances to the load. A great disadvantage of such systems is their low efficiency, so that the cost of supplying energy to complicated loads is enormously increased.
A further problem with uninterruptible power supplies arises when the system being powered requires many different power levels. Heretofore, it has been thought that direct connection of the power supply load to a battery of appropriate voltage was too inefficient to be feasible. It was further thought that battery power is not acceptable because systems such as control instrumentation require a highly regulated power supply.
Accordingly, battery backup systems have been proposed which provide individual battery backup circuits for each subcircuit in a complex system as found in a computer. An example of such multiple backup system arrangement is disclosed by Graf et al in U.S. Pat. No. 4,143,283. Graf et al disclose essentially four independent circuits, one for each voltage used by the device, and use relays to switch batteries from parallel to series as needed for each voltage. User intervention is required to suspend functions, which is undesirable. Furthermore, redesign, repair, or the like are very difficult and expensive with a system which uses separate backup circuits for each voltage level used in the device. Still further, a Graf, et al type device may require a plurality of expensive transformers.
Thus, in addition to the problems associated with uninterruptible power suppies due to their designs, such supplies have other drawbacks when used in conjunction with systems having a plurality of different voltage levels.
For these reasons, uninterruptible power supplies using batteries, even parallel connected batteries, have not been thought suitable for systems in which a variety of voltages are required.
It is also desirable to provide an uninterruptible power supply which is a general purpose device. That is, it is desirable for one uninterruptible power supply to be adaptable to a wide variety of uses, by a wide variety of users, especially where DC current is the desired current to ultimately power the solid state electronics. By attempting to apply a single uninterruptible power supply to a general use, all of the above-discussed problems and drawbacks associated with such devices are exacerbated. In addition to this, such a goal creates further problems as such a device should be as easy to use as possible to the largest number of people possible can use it; therefore, such a device should not have complex controls, nor should it have complicated control signals. It should also be modular to be easily installed in and removed from a wide variety of devices without requiring substantial modification of equipment. Such a general purpose device should also be designed so as to place as few restrictions as possible on the loads it is used with.
Furthermore, the general purpose device should be amenable for use directly with a variety of loads, and should be amenable to mass production. In order to be economical to produce and use, such a system should be battery operated so that heretofore known battery backup power systems are not suitable for such use due to the problems discussed above in connection with battery systems. Still further, the battery of such a general purpose system should be rechargeable and amenable to being rapidly recharged when it is in a standby mode. Additionally, such a battery should be easily replaceable, even while the system is running, if desired.
So that an operator of the protected device will know when the reserve power supply is standing by or when it is providing power, it is necessary to include means within the uninterruptible power supply unit whereby audible and/or visual signals can be generated. It is also desirable to provide a low or out of battery power signal whereby an operator is warned that the backup power supply is low or exhausted and/or automatic action is taken either when reserve power is low or exhausted.
At present in the design of computers the emphasis often lies on busses for (DC) power between the various units of the system. This is because a buss is a very attractive proposition for the designers of computers in view of the large number of peripheral units that can be connected thereto, the designer thus also having a freedom in choosing the desired configuration. Such a buss permits a high flexibility in the design of the system. One of the possibilities of such a system includes the parallel connection of all peripheral units to the said buss, each of these peripheral units being connectable to any other unit.
The use of a buss appears to be appropriate to produce a general purpose uninterruptible power supply in which a variety of loads are protected. Furthermore, using a circuit board concept, such as disclosed in patents such as U.S. Pat. No. 4,151,580 and others, a modular uninterruptible power supply could be provided. However, the above-discussed problems associated with uninterruptible power supplies, and especially those problems associated with uninterruptible power supplies using batteries, have prevented the adaptation of a buss concept to an uninterruptible power supply which is modular and capable of wide, general use.